That is the entire point of a low-pass filter!
The low frequencies pass right through.
Frequencies above the passband edge (-3dB point) are attenuated more than 3dB.
It looks like the passband edge for this filter is at around 1200 to 1500 Hz.
Likely a Sallen-Key/Butterworth implementation.

The primary reason for the gain being a function of frequency is that the circuit appears to have a component whose impedance is a function of frequency. The most common components that have this characteristic are inductors or capacitors.